A standard light source is one of the most important standards for the measurement of light. Prior to delivery from the factory or before being put to actual use, the readings for precision optical instruments (such as illuminance meters, color analyzers, etc.) must be calibrated using the standard light sources. Illuminance meters, for example, must in accordance with national standards undergo calibration with CIE standard light source A.
Many nations have their National Measurement Laboratories (NML) capable of providing standard light sources. Examples given are: the National Institute of Standards and Technology (NIST) in the USA and National Physical Laboratory (NPL) in Britain. But due to the limited useful life of these standard illuminants produced, they are not practical for use as calibration instruments. In typical application, the standard illuminant used in industry is reproduced from these laboratories' standard light source.
In other words, the standard light source (standard lamp) which users purchase from the above-mentioned laboratories emits a certain color temperature and luminous intensity when supplied with a certain level of electric voltage (or current). The color temperature and luminous for these standard illuminants have been precisely measured and have the value noted by the laboratories that produce them. By referring to the values for color temperature and luminous intensity at certain voltages (or currents) of the original standard light source, users can use a reproduction apparatus to run an electrical current through a tungsten lamp (reproduced lamp) and, by adjusting voltage (or current), cause the reproduced lamp to emit light with a color temperature identical with that of the original standard light source.
Next, by using the principle of "the inverse-square law of illumination" it is possible to calculate the luminous intensity of the tungsten lamp (reproduced lamp) based on the luminous intensity of the original standard light source (known values) and the distances between the detector and both light sources, respectively. The color temperature and luminous intensity of the tungsten lamp at a certain voltage can thus be determined, and, thus, the tungsten lamp can then be used as a standard light source. This process for reproducing a standard light source is wildly used by many standard lamp calibration laboratories.
As mentioned previously, the color temperature and the luminous intensity are the two key parameters for standard light sources. The color temperature is defined as follows: "The temperature of the a full radiator which emits radiation of the same chromaticity as the radiation considered." Luminous intensity is defined as: "The luminous flux per unit solid angle in the direction in question."
To illustrate relations between current, color temperature, and luminous intensity, three 500 W standard lamps (with C-13B tungsten filaments) are given in Table 1 as examples:
TABLE 1 ______________________________________ Corre- Lamp Current sponding Luminous Color number (A) voltage(V) intensity(cd) temperature(K.) ______________________________________ NBS9721 4.021 112.7 746 2856 NBS9720 4.000 112.3 753 2856 NBS9890 4.046 113.4 756 2856 ______________________________________
Table 1 shows that at a designated current (for instance, 4.021 amperes for the NBS9721) these standard lamps emit light of a designated color temperature (2856K) and a designated luminous intensity (746 candle) in a designated direction.
Because color temperature and luminous intensity are the two key parameters for standard light sources, the conventional reproduction of same color temperature luminous intensity standard light source requires the use of two apparatuses (two steps): one for adjusting color temperature, and another for measuring luminous intensity.
FIG. 1 is a flow chart for the traditional reproduction procedures for a same color temperature luminous intensity standard lamp. Prior to the reproduction process, a lamp which will be reproduced to be a standard light source is prepared and the color temperature and luminous intensity of a standard lamp are obtained. The first step in the reproduction process is to use a color temperature adjusting apparatus to adjust the color temperature of the reproduced lamp to that of the standard lamp.
At (20) the standard lamp is installed in the color temperature adjustment apparatus; at (21) the standard lamp is supplied with a designated current so that it emits light with characters identical to the values as given in the table 1; at (22) the red-filtered (approximately 650 nm) and blue-filtered (approximately 450 nm) detectors of the color temperature adjustment apparatus are installed at a set distance and aimed at the standard light source and the output signal intensities of the red-filtered and blue-filtered detectors are measured; at (23) the measured value of the red-filtered detector is divided by that of the blue-filtered detector to obtain the red/blue signal ratio R1. Next at (24) the standard lamp is replaced with the reproduced lamp; at (25) the input voltage to the reproduced lamp is adjusted to a work voltage; at (26) the signals at the red- and blue-filtered detectors are measured, and; at (27) the measured values are calculated to obtain red/blue signal ratio R2. At (28) the input voltage to the reproduced lamp (or current) is adjusted to where the red/blue signal ratios R2 is equivalent to R1, and at (29) the input current I and voltage V of the reproduced lamp is recorded. This completes the first stage, the color temperature adjustment procedure for the reproduced lamp.
The reproduced lamp completed through the above procedure will in theory have the same color temperature as the standard light source if there is no non-linear problem in the red-and blue-filtered detectors of the color temperature adjustment apparatus.
The next step is the measurement of the luminous intensity of the reproduced lamp. At (30) the standard lamp is installed with the luminous intensity measurement apparatus; at (31) the input current to the standard lamp is adjusted to a designated value so that it emits light with a designated luminous intensity I.sub.s ; at (32) the detector of the intensity measurement apparatus is installed at the distance r.sub.s from the standard lamp and the output signal value S.sub.1 of the detector is measured; at (33) the standard lamp is replaced with the reproduced lamp; at (34) the input current to the reproduced lamp is adjusted to the current I obtained at (29); at (35) the distance between the detector and the reproduced lamp is adjusted to where the output signal value S.sub.3 of the detector is equal to the value S.sub.1 arrived at (32). At (36) the distance from the reproduced lamp to the detector r.sub.t is recorded. From the known luminous intensity I.sub.s of the standard lamp, the luminous intensity I.sub.t of the reproduced lamp can be calculated by using the "inverse square law of illumination"; thus: EQU I.sub.t =I.sub.s * ((r.sub.t).sup.2 /(r.sub.s).sup.2)
Where r.sub.s and r.sub.t are distances arrived at (32) and (36) respectively. When the above procedures are completed it may be found that the luminous intensity of the reproduced lamp is I.sub.t when it is supplied with current I arrived at (29), and color temperature of the reproduced lamp is identical with that of the standard lamp. Users can use the above results to employ the reproduced lamp as a standard lamp.
From the above description several shortcomings can be seen in the conventional method for reproducing a standard lamp:
First, the conventional approach requires that the color temperature adjustment apparatus and the luminous intensity measurement apparatus be employed separately in two steps, i.e., to adjust color temperature and to measure luminous intensity, respectively, in order to reproduce a standard lamp. The procedures involved are extremely troublesome and time-consuming.
Also, the accuracy of the color temperature of the reproduced light source relies upon the accuracy of the red- and blue-filtered detectors. In other words, accurate adjustment of the color temperature of the reproduced lamp can only be obtained with apparatus in which the intensity of the output signals of the detectors are in linear relation with the illuminance they receive.
Below are described a series of situations in which color temperature adjustment error occurs due to failure to achieve linear relation between the intensity of the output signals of the detector and the illuminance they receive. With a tungsten filament lamp the spectral power distribution property is similar to that of a blackbody, which means that the color temperature of a tungsten filament lamp can be expressed using the ratio of spectral radiances at any two wavelengths (it is typically expressed as the ratio of spectral radiances at approximately 650 nm (blue) wavelength and at approximately 450 nm (red) wavelength). As for the detectors, the parameter of light it detects is the irradiance (or illuminance). The irradiance at wavelength E.lambda.=I.lambda./r.sup.2 (or E=I/r.sup.2), wherein I is the radiant intensity at wavelength.lambda. of the light source, and r is the distance from the light source to the detector. Thus the color temperature T.sub.c of the light source can be expressed as the ratio of spectral irradiance of the light source at distance r: EQU T.sub.c =(I.sub.650 /I.sub.450)=(E.sub.650 /R.sup.2)/(E.sub.450 /R.sup.2)=(E.sub.650 /E.sub.450)
where I.sub.650 and I.sub.450 are the spectral radiant intensities at wavelengths 650 nm and 450 nm, and E.sub.650 E.sub.450 are the spectral irradiances at the wavelengths 650 nm and 450 nm. Assuming the standard light source of color temperature T.sub.c is located at distance r, its spectral irradiance at wavelengths 650 nm and 450 nm are E.sub.650 and E.sub.450, and the output signal intensities of the red- and blue-filtered detectors are S.sub.650 and S.sub.450, the standard light source's color temperature as measured by this instrumentation can be expressed as R=S.sub.650 /S.sub.450. To adjust the color temperature of the reproduced lamp to be equivalent with that of the standard lamp, the voltage supplied to the reproduced lamp in the color temperature adjustment apparatus is adjusted to where the ratio of the output signal intensities of the red-and blue-filtered detectors equals R as measured to the standard light source. Now assuming that this adjustment has been completed, and that the signals detected by the red-and blue-filtered detectors are aS.sub.650 and aS.sub.450 respectively, the spectral irradiance of the reproduced illuminant at 650 nm and 450 nm should be aE.sub.650 and aE.sub. 450, if the output signal intensities of the detectors are linear with the spectral irradiances they receive. The ratio between the red-and blue-filtered spectral irradiance of the reproduced illuminant is therefore equal to that of the standard illuminant, and accurate color temperature adjustment is obtained. If, however, the output signal intensities of the detectors are not linear with the spectral irradiances (or illuminance) they receive, then even if the same signal output ratio R is obtained and the signal output is also aS.sub.650 and aS.sub.450, the relative spectral irradiances could be bE.sub.650 and cE.sub.450, and c may not equal b. Thus the adjusted color temperature may not be equal to the color temperature T.sub.c of the standard lamp, and error will occur. This type of error is especially apparent when there is a relatively larger difference in luminous intensity between the reproduced lamp and the standard lamp.
Furthermore, the traditional method for preparing a standard lamp requires a large amount of manpower and time. Both the voltage adjustment control to the lamp as well as adjustment of the distance between the lamp and the light detectors both must be performed manually. Also, due to the high precision required, operators must spend long hours to complete the reproduction procedure.
There is thus a need to have an automated apparatus and method for the reproduction of same color temperature luminous intensity standard lamps. Said apparatus and method could simplify procedures in the reproduction of standard lamps, thereby reducing man hours and improving reproduction precision.
An object of this invention is to lower costs of manual operation and to reduce human-induced error by providing an automated apparatus and method for the reproduction of same color temperature luminous intensity standard light source.
Another object of the invention is to reduce the number of steps in the reproduction of a standard lamp by providing a method for reproducing a same color temperature luminous intensity standard light source.
Another object of the invention is to provide an apparatus and method which can eliminate the effects of non-linearity in the filtered detectors and achieve precise color temperature adjustment during the reproduction of a same color temperature luminous intensity standard light source.
Another object of the invention is to provide a one-set apparatus and a one-step procedure for the reproduction of standard light source.